Hypothalamic regulation of pituitary gonadotropin secretion in the male rat differs from that in the female and these differences are established postnatally under the influence of testicular androgen. Similarly, the neural control of sexual behavior and of food intake are under the influence of androgen postnatally. Studies will be conducted: 1) to elucidate the possible mechanisms of the permanent organizing action of gonadal steroids (androgen and estrogen) on the hypothalamus by means of the systemic or intracerebral administration (and timed removal) of these steroids and potential antagonists including antibiotics; 2) to elucidate the role of extrahypothalamic input in the regulation of pituitary function and sexual behavior in the male and female; 3) to determine the influence of small surgical damage within the basal hypothalamus on its function, including the regulation of food intake and body weight. These studies will include stereotaxic procedures in the newborn rat. In addition, studies of adult neural-gonadal interrelations will be undertaken to elucidate the mechanisms of action of hormones on the brain. Several model systems will be studied: hormone induced sexual behavior, hormone induced changes in the electrical activity of the brain, hormone induced changes in pituitary activity, and hormone induced changes in food intake. Techniques will include the direct application of hormones, drugs, and antibiotics to specific loci throughout the brain, electrolytic lesioning and stimulation, and the technique of surgical deafferentation of the hypothalamus. A technique which permits direct visualization of the basal hypothalamus will allow the accurate placement of small lesions and knife cuts within this important region of the brain.